20.2.5 Sync-E

<Structure of this section>

(1) Overview

Sync-E(Synchronous Ethernet) is a technique for synchronizing the clock frequency used in PHY(Physical Interface) with an Ethernet interface. The master-slave network with the primary reference clock (PRC:Precision Reference Clock) as its vertex synchronizes the clock frequency, and the Ethernet clock of all the devices in the system refers to a single clock. This allows network devices to communicate with each other at the same clock frequency and improves the transfer performance of the entire system.

Synchronization by Sync-E can be done either by synchronizing to the signal received on the port or by synchronizing based on the time stamp on the frame. The Switch uses a method to synchronize the signal received on the port.

If you specify a port that receives an external clock in the configuration, other ports operate at the frequency of the external clock. Specify SFP+/SFP shared port as the receive port for the external clock. If Sync-E is disabled, such as when there is no external clock input or when the configuration does not specify a port for receiving the external clock, the Switch operates with the internal clock. The following diagram outlines Sync-E.

Figure 20-6: Networking with Sync-E applied

In this configuration, the external clock and clock frequency are synchronized and synchronized with the downstream unit at the synchronized clock frequency. When the clock is out of synchronization with the external clock, it operates with the internal clock.

(2) Support Specifications

Sync-E of this device complies with ITU-T G.8261 and G.8262. The following tables show the supported status of Sync-E.

Table 20-17: Supported status of Sync-E
Item	Specifications
Supported models	AX3660S-24X4QW AX3660S-48X4QW
Interface that can receive external clock*1	10GBASE-R (SFP+) 10GBASE-CU (SFP+)
Interface to which an external clock can be applied*2	10GBASE-R (SFP+) 10GBASE-CU (SFP+)

Note #1: Management port and QSFP28/QSFP+ shared port are not supported. If a SFP or SFP-T is used on a SFP+/SFP shared port, clock synchronization on that port is not guaranteed.
Note #2: Management port is not supported. If a SFP or SFP-T is used on a QSFP28/QSFP + shared port or a SFP+/SFP shared port, the operation of clock synchronization on that port is not guaranteed.

(3) Basic operation

Depending on the operation status of Sync-E, different clocks are used.

Sync-E is enabled: All ports operate with the external clock when synchronization with the external clock is performed on the specified receive port. When synchronization with the external clock is lost on all the specified reception ports, operation is performed using the internal clock.
Sync-E disabled: Operates with the internal clock.

(4) Clock switching operation

In the Switch, up to two Sync-E external clocks can be specified with priorities, and up to three clock sources in combination with the internal clocks can be used to select synchronization destinations. The clock sources with the highest selection priority are as follows:

High priority external clock
Low priority external clock
Internal clock

If synchronization is lost due to a failure of the clock selected as the clock source, the clock with the highest priority is automatically switched immediately to the clock that is synchronized at that time. If a clock with a lower priority is selected as the clock source and synchronization of a clock with a higher priority is performed, the clock is automatically switched after the time to suppress automatic switchback has elapsed. The following tables show how clocks are switched on Sync-E.

Table 20-18: Clock Switching Behavior in Sync-E
Trigger for switching	Synchronization destination before switching	Synchronization destination after switching	Switching operation
When the selected clock source is out of sync	High priority external clock	Low priority external clock	Switched automatically immediately
	High priority external clock	Internal clock
	Low priority external clock	Internal clock
Synchronization with a clock with a higher priority than the selected clock source occurs.	Low priority external clock	High priority external clock	Switches automatically after the switch-back suppression time has elapsed.
	Internal clock	Low priority external clock
	Internal clock	High priority external clock

(5) Displaying Sync-E Status LED

The external clock status can be checked using ST2 LED on the front of the system. However, ST2 LED during stack configuration displays the stack status. For the status and meanings of ST2 LED, refer to the "Hardware Instruction Manual".

(6) Prohibited configuration

When specifying an external clock, do not configure the synchronization destination as the synchronization source. In such a configuration, the external clock cannot be synchronized correctly.

Figure 20-7: Sync-E Disable Configuration Example 1 (Configuration Specified as External Clocks in Both Sections)

Figure 20-8: Sync-E Disable Configuration Example 2 (The external clocks specified by the Device are synchronized to the Switch)

(7) Notes

External clock synchronization through the stack port is not supported. Specify an external clock for each member switch.
To synchronize the primary reference clock to the terminal, all devices in the network must support Sync-E because they are synchronized on the master-slave network.
Selects a synchronized port as the clock source, regardless of the link state of the port. Therefore, even if a protocol-based route switching occurs, the external clock may not be switched.